I need to do FEA for 2 circular mating parts (One circular part seated in a pocket/counterbore in the another part). I have following questions:

1. i have to simulate no pentration contacts between the parts. Can i give Global no penetration component contact or should i individually give No pentration contacts set between the touching faces. Which approach is better.

2. Gap between the outer dia. of one part and inner dia. of another part is .015". I am not very sure if these two faces will touch each other under the action of applied load.Should i contacts at the non touching faces??

1) The Global Contact will only apply to surfaces that are initially touching. If you have a gap between them they do not automatically have any constraints so you will need to add no penetration to those pairs of surfaces. If they are already touching then there is no difference, the Global Contact simply makes it faster by automatically applying this condition to all touching faces.

2) If they have the potential to touch due to deformation it is better to add the no penetration contact. If they do not deflect far enough to touch it does not hurt, it is simply a few seconds of wasted time to add that contact. Better to waste time and know the results are accurate than save a few seconds but wonder about the validity of the results.

Suppose i dont give "No penetration" contacts between two non touching surfaces, what will the software consider in case the two surface touch each other during the FEA run under the action of applied load.

If you do not provide a no penetration contact then the software will let the two parts occupy the same space at the same time. It will ignore the fact that they would touch and instead tell you what the displacement of each would be if the other was not there.

If I remember correctly there is a setting for defining the distance between parts for it to automatically apply a no penetration. In this case it would be that your 2 parts are close enough that it automatically applies the condition for you. (I'm limited to 2011 but took the simulation class in 2013 so I get confused over which features are available when).

probably an issue with characteristic length, check it out in the kb pablo.

basically the gap gets ignored because of the overall size of the geometry relative to the mesh size. as such it looks like something that should be separated acts bonded because they are essentially merged. can also show similar behavior with no pen where the gap never gets bridged.